2016ApJ...830..148C

We report the discovery and analysis of the most metal-poor damped Lyα (DLA) system currently known, which also displays the Lyman series absorption lines of neutral deuterium. The average [O/H] abundance of this system is [O/H] = -2.804 ± 0.015, which includes an absorption component with [O/H] = -3.07 ± 0.03. Despite the unfortunate blending of many weak D i absorption lines, we report a precise measurement of the deuterium abundance of this system. Using the six highest-quality and self-consistently analyzed measures of D/H in DLAs, we report tentative evidence for a subtle decrease of D/H with increasing metallicity. This trend must be confirmed with future high-precision D/H measurements spanning a range of metallicity. A weighted mean of these six independent measures provides our best estimate of the primordial abundance of deuterium, 10⁵ (D/H)_P = 2.547 ± 0.033 (log₁₀ (D/H)_P=-4.5940±0.0056). We perform a series of detailed Monte Carlo calculations of Big Bang nucleosynthesis (BBN) that incorporate the latest determinations of several key nuclear reaction cross-sections, and propagate their associated uncertainty. Combining our measurement of (D/H)_P with these BBN calculations yields an estimate of the cosmic baryon density, 100 Ω_B,0 h²(BBN) = 2.156 ± 0.020, if we adopt the most recent theoretical determination of the d(p,γ)³He reaction rate. This measure of Ω_B,0 h² differs by ∼2.3σ from the Standard Model value estimated from the Planck observations of the cosmic microwave background. Using instead a d(p,γ)³He reaction rate that is based on the best available experimental cross-section data, we estimate 100 Ω_B,0 h²(BBN) = 2.260 ± 0.034, which is in somewhat better agreement with the Planck value. Forthcoming measurements of the crucial d(p,γ)³He cross-section may shed further light on this discrepancy.